3 weeks sure, but I'd argue that if you're a fast typist, it'll be more than three weeks and likely a few months before you reach your old high speeds. Either way, I'm still against switching one key at a time.
Weird question, do people tend to use just one of the Shift keys? I'm a lefty and I've only ever used my Left Shift. In a way, it feels like it helps balance out my Dvorak use.